THEORETICAL FOUNDATIONS OF FISH FARMING - I.M. Sherman - 2011

2. THEORETICAL FOUNDATIONS OF THE FORMATION AND UTILIZATION OF FISH PRODUCTIVE TRAITS

2.5. Stages of Fish Development and Their Application in Fish Farming

Ensuring high profitability in fish production requires a profound understanding of the regularities governing The Development of aquaculture species, as well as an appreciation of their biological characteristics across various life stages. Equally important is a thorough grasp of all technological processes without exception—particularly artificial reproduction, including the ecological conditions for maintaining broodstock, stimulation and maturation, gamete collection, egg insemination and Fertilization, incubation, free embryo maintenance, and the rearing of hardy juveniles.

To date, the most comprehensive theory illuminating the fundamental patterns of development—grounded in The Unity of the Organism and the aquatic environment and in the adaptive Nature of the developmental process—is The Theory of developmental stages. Formulated by V.V. Vasnetsov in the 1940s and 1950s, it has lost none of its significance to this day. He demonstrated that the entire development of a fish represents a consecutive series of stages, each characterized by Specific features of Structure, physiology, and ecology. This theory is based on a morpho-physiological and ecological analysis of specific intervals of fish development encompassing nearly all Organ Systems. V.V. Vasnetsov and his followers studied not only the shape and function of the body and fins, cutaneous Sensory Organs, the oral and pharyngeal apparatus, and the Digestive System across successive developmental stages. Every moment of fish development was considered a developmental stage. Because the most indicative criterion was continuous linear growth, larvae were studied at intervals of every tenth of a millimeter. Regarding changes in fish ecology, the authors recorded the embryos' relationship to dissolved oxygen—expressed through Circulatory system features and embryonic movement—and, for larvae and older developmental stages, primarily shifts in feeding, the quality of food items, or their relative sizes. Certain characteristics were deduced by examining anatomical structures. For instance, a superior Mouth combined with specific fin structures indicated surface-feeding habits, whereas an inferior mouth and a particular body shape pointed to bottom-feeding. This methodological and systematic approach allowed researchers to clearly observe changes in fish development.

To illustrate what developmental stages entail, consider the following example. Upon hatching from the membrane, the embryos of phytophilic fish, such as bream, begin to swim and attach themselves to submerged vegetation. At this point, they live outside the membrane, and their ecology has changed significantly. This represents a new phase in their life activity compared to the Embryonic Stage occurring inside the membrane. Meanwhile, the embryos grow and develop, gradually consuming their yolk reserves. However, at this stage, the yolk lacks sufficient nutrients for intensive organismal GROWTH AND DEVELOPMENT, creating a need for supplementary exogenous feeding. To secure this, pre-larvae detach from the plant substrate, begin to swim, pursue prey in the form of small planktonic organisms, catch them, and digest them. This new phase in fish biology (beginning with the onset of exogenous feeding, termed the larva) is referred to the mixed-feeding stage, since larvae consume external food alongside their residual yolk.

Subsequently, the larvae exhaust their yolk reserves entirely and switch exclusively to an external (exogenous) feeding type, thereby initiating a new phase—purely exogenous feeding. The authors of these studies emphasize that each biological feature corresponds to a specific structural trait and, consequently, a specific function in the fish.

At the boundaries of developmental stages—during transitions from one stage to the next, which occur abruptly—primordia of new organs and new Functions of modified organs emerge. For example, in certain cyprinid Fishes, the rudiments of the pelvic fins appear during the transition from the sixth to the seventh larval stage. Here, these rudiments assume a new balancing function. Throughout this developmental stage, the balancer rudiments gradually increase in size and are reinforced first by mesenchymal rays and later by ossifying rays, as the fish grows intensively and requires more robust stabilizers. At the moment of transition to a new developmental stage, they alter their function, acquiring the capacity for active movement.

It is also noteworthy that groups of developmental stages united by a common adaptation constitute developmental periods. Accordingly, researchers distinguish the embryonic, larval, fingerling, and sexual maturity periods in fish. A characteristic feature of the embryonic developmental period—and a general one for it—is that the respiratory organs are Blood Vessels located on the yolk, on the embryo's body, and sometimes on the fins. With the transition to the larval period, branchial respiration emerges, while only remnants of the previous Respiratory system are retained. Common to all Stages of the larval period is that fin folds serve as locomotor organs, whereas for the fingerling period, it is the presence of many adult-like traits coupled with the absence of sexual maturity. Finally, a series of features characterizes the stages of the sexual maturity period, chief among them being sexual maturity itself.

It should be noted that V.V. Vasnetsov focused primarily on the transitions from one developmental stage to another and on the saltatory (abrupt) nature of these transitions. Meanwhile, all processes occurring throughout a given stage were termed the developmental interval, during which only slow, gradual, almost imperceptible changes take place. Nevertheless, the theory of stages continued to be deepened and developed by V.V. Vasnetsov’s followers. S.G. Kryzhanovsky established that quantitative as well as qualitative changes occur at every moment of development—that is, at every developmental stage. Consequently, all prerequisites for transitioning to a new stage are created during the preceding stage, which reinforces METABOLISM/2.html">THE CONCEPT OF the unity of the organism and its environment. Indeed, every moment of metabolism induces both quantitative and qualitative changes. It must also be remembered that growth, which is inextricably linked with the organism's development and constitutes its quantitative aspect, is the result of this metabolism.

Growth is likewise both gradual and abrupt precisely because it is the result of every moment of metabolism.

As confirmation, we emphasize that the consumption of even a minute fraction of the animal's own yolk—even in a negligible amount—alters its shape, size, and mass, thereby changing the organism's relationship with the aquatic environment. The exact same picture can be envisioned for every function of the animal.

Proceeding from this, questions concerning the explanation of changes occurring during the organism's development are of paramount interest.

More detailed studies have revealed that throughout a developmental stage, quantitative and qualitative changes in structure—and consequently in the Functions of the animal organism—accumulate gradually and sequentially, along with minute alterations in environmental relationships. The accumulation of these changes continues up to a certain limit. Upon reaching this threshold, a transition occurs to a new qualitative state of environmental relations—a transition to a new developmental stage—which corroborates the philosophical Concept of the transformation of quantity into quality. Regarding this approach to these changes, S.G. Kryzhanovsky made a rather original remark, stating that "...the final changes that determine The Emergence of a new quality and the transition to the next developmental stage may be as insignificant as the drop that overflows the cup."

The developmental stages that comprise developmental periods also change sequentially. Upon reaching a certain limit of these changes, a transition takes place to a new and larger scale of ecological quality—one that transcends the stage-to-stage transition—namely, the transition to a new developmental period. Examples include the transition from the larval to the fingerling period, and from the fingerling period to the sexual maturity period, etc.

A positive role in the advancement of the general theory of stages was played by S.G. Kryzhanovsky’s theory of ecological groups of fish, which made it apparent that the adaptive nature of various stages varies across species and ecological groups. The number of developmental stages comprising developmental periods was found to differ. For instance, in the pike, only

three larval developmental stages were identified, whereas lithophiles exhibit four, and phytophiles six. In lithophilic salmonid fishes, the larval developmental period can essentially be viewed as a prolonged stage of mixed feeding on endogenous yolk and exogenous food. In phytophilic cyprinids, this stage is quite short, with juveniles feeding externally throughout almost the entire larval period. In salmonids, a spawning developmental stage has been discovered, characterized by the reorganization of their adaptation system associated with moving to new, favorable spawning grounds in the Water body.

A great diversity of respiratory adaptations is observed among fish embryos in nature; these change during ontogeny, and oxygen requirements differ substantially among species. Data exist regarding regular shifts in respiratory function during embryonic development across several species representing different ecological groups.

It has also been established that the average size of fish at each developmental stage fluctuates within a certain range specific to each species. However, under significant environmental alterations, fish at later developmental stages may be smaller than those at earlier ones. Therefore, characterizing a stage solely by fish size is insufficient; deeper and more detailed studies are necessary to gather information for a comprehensive characterization of developmental stages.

The duration of developmental stages depends on the environment, particularly the availability of necessary food, water chemical composition, Temperature, and other conditions. Consequently, in different parts of a species' range, coeval juveniles may find themselves at different developmental stages.

Discrepancies in timing, size, and morphological features during stage transitions within the same species under varying conditions do not contradict the theory of developmental stages; rather, they emphasize the adaptive nature of the organism's developmental process.

Based on the foregoing, it is appropriate to examine the periods and stages of fish development.

The THEORETICAL FOUNDATIONS OF fish developmental stages, formulated by V.V. Vasnetsov, boil down to the fact that throughout various periods of ontogeny, fish development proceeds not only gradually and continuously, but also discontinuously and saltatorily (abruptly). This is accompanied by sharp changes in The structure of organ systems, which occur quite rapidly—sometimes within a few hours or even minutes. These morphological changes are inextricably linked with shifts in the biological CHARACTERISTICS OF THE fish. Between such abrupt changes, gradual growth and sometimes barely perceptible quantitative changes take place. All these changes occur within certain limits, so they do not alter the core quality characterizing a given interval of fish development. That is, the qualitative Features of the organism and its relationship with the external environment remain unchanged. Such intervals of relative stability in fish development between two abrupt changes are termed stages.

In turn, each stage of fish development is characterized by specific structural features, biological traits, and environmental requirements. In the absence of necessary conditions, fish are delayed at a given stage. In doing so, they slow down or completely cease their growth and perish.

Drawing upon theoretical foundations, fish development according to Vasnetsov's theory proceeds both gradually and discontinuously (saltatorily), breaking down into a series of consecutive stages characterized by growth and gradual changes, yet lacking any fundamental alterations in the structure, physiology, or biology of the fish organism. Fish development is a sequential succession of organismal adaptations to the external environment across specific stages.

The progression of certain stages in various fish species is not uniform; they vary in duration and frequently exhibit specific features of their own. This Conclusion is critically important for developing biotechnological Methods in artificial fish breeding. It highlights the necessity of studying the biological and

Ecological features of the Developmental Stages of each fish species targeted for artificial propagation.

Periods and stages of fish development are characterized by distinct morphological changes, which necessitates a thorough Study of the morphophysiological patterns governing early fish ontogeny.

It is well known that population size is largely determined by the peculiarities of early fish ontogeny. Therefore, one of the most vital aspects for devising strategies for the further development of fisheries is establishing the patterns of productivity in communities or individual fish populations, alongside a comprehensive understanding of the morphophysiological features of their individual development.

Research by V.V. Vasnetsov, S.G. Kryzhanovsky, and L.P. Ryzhkov has established that a specific interval of fish development is a stage during which growth and gradual qualitative changes occur, creating the necessary prerequisites for transitioning to new stages within a relatively short period, albeit—as mentioned above—in a saltatory manner. Studies have also proven the existence of periodicity in the changing rates of organism differentiation. Specifically, it has been revealed that an increase in the rate and number of differentiations is typically observed during the transition from one developmental stage to the next, i.e., in the so-called transitional phase, whereas their slowdown occurs throughout each respective stage. When examining the ontogeny of individuals of a certain species—a universal phenomenon—four distinct periods are distinguished: embryonic, larval, juvenile (fingerling), and adult. These are characterized by specific morphological, ecological, and physiological features. Specifically, the Embryonic period is defined by the embryo residing within the egg membrane; the larval period represents the larval stage with endogenous, mixed, and purely exogenous feeding until the appearance of the central scales plates. The juvenile period begins with The formation of the first circuli (sclerites) on the scales and lasts until the onset of sexual maturity, while the adult period commences at sexual maturity and continues until the end of The life cycle. Each of these periods is subdivided into a series of stages featuring specific morphophysiological and biochemical characteristics.

The number of stages in various fish species can vary and depends on their biology. For instance, seven stages have been identified in the larval period of salmon, and eight in the Sevan trout, driven by the biological peculiarities of these species.

Each stage, in turn, is subdivided into phases, which provide a concrete characterization of a particular morphological and physiological state of the fish organism. Based on this, the transitional phase can be viewed as the concluding part of each stage, which, As a result of complex morphophysiological transformations, ensures the transition to the subsequent developmental stages.

Special ecophysiological and biochemical studies have established that most fish species during ontogeny exhibit periodic changes in PHYSIOLOGICAL AND BIOCHEMICAL processes—particularly in oxygen consumption intensity, i.e., gas exchange—which are closely linked to the morphological transformations of the organism. The Essence of this phenomenon in early ontogeny lies in a significant surge in its rate during transitional phases and its reduction throughout all developmental stages. Each developmental stage is characterized by specific limits and average levels of gas exchange. Another characteristic feature is the increase in oxygen consumption intensity from the moment of egg fertilization until the onset of larval feeding, followed by a subsequent deceleration as the juvenile fish continue to develop.

Alongside this, a regular periodicity in the changes of linear growth and weight accumulation in fish has been uncovered. This is determined and diagnosed by a pronounced acceleration at the beginning of each developmental stage, followed by a deceleration throughout the stages, reaching a minimum during the transitional phase. Despite some slowdown in organism growth rates during the stages, each developmental stage possesses specific limits of variation and average rate values, which are conditioned by the morphophysiological characteristics of the organisms during that given developmental period, as well as species-specific traits of the fish.

A general pattern is that each qualitatively new developmental stage begins with a significant surge in growth rate, which subsequently also demonstrates a slowdown. At the same time, at the beginning of a stage, the gas exchange intensity relatively stabilizes at a level characteristic of that respective stage. Its value is 1.5–3.0 times lower than during the transitional phase. Throughout the stage, The rate of differentiation is negligible. Toward the end of the stage, both the growth rate and gas exchange intensity reach maximum values, although the differentiation rate may increase slightly. Overall, the organism at this time is prepared to transition to a new developmental stage. As a result of morphophysiological and biochemical transformations during the transitional phase, qualitative changes occur within the organism as it advances to a qualitatively new stage. The transitional phase begins with a sharp increase in oxygen consumption intensity, which provides the necessary energetic capacity for morphological and other transformations. Upon reaching the maximum respiration level, the rate of differentiation increases intensively. At this time, the linear and weight growth of the organism reaches its minimum. All the energetic resources of the organism are directed toward rapidly executing the transitional phase, during which a rather unstable interaction with the external environment is recorded. Toward the end of the transitional phase, the gas exchange intensity and differentiation rate slow down and relatively stabilize by the beginning of the stage, while the organism's growth rate increases. The changes occurring during the developmental stage and the transitional phase are presented in Figure 2.8.

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Fig. 2.8. Changes in growth rate and gas exchange intensity upon transitioning to new developmental stages (after L.P. Ryzhkov, 1984)

1 - gas exchange intensity; 2 - growth rate; 3 - differentiation rate.

An in-depth study of the periodicity of organism differentiation processes, growth rates, and gas exchange intensity holds not only theoretical interest but also practical significance for investigating the productivity of biological systems and the rational utilization of their output under conditions of organizing research at a qualitatively new energetic level.

During fish development up to the attainment of sexual maturity, an increase in the absolute indicators of plastic and functional metabolism has been established. This may be driven by the overall increase in fish body mass, which demands heightened energy resource requirements throughout development—both for building the organism's body and for executing core life-support processes. A general pattern for most freshwater fish is a substantial increase in the rate of functional metabolism compared to plastic metabolism. Changes in the values of functional and plastic metabolism are closely interconnected both with each other and with the intensity of morphogenesis processes, as it is well known that morphological transformations occur most intensively during the transition from one developmental stage to another—namely, during the transitional phase.

It has been established that upon transitioning to a new developmental stage, There is a sharp and typically significant increase in the utilization of transformed

food energy toward functional metabolism, which supplies the necessary energy for morphological transformations, as evidenced by the increasing rate of differentiation and morphological changes.

The biological expediency of increasing Energy Expenditure on functional metabolism lies in the necessity to rapidly ensure the organism's transition into a qualitatively new state, thereby eliminating the minor disequilibrium that arose between the organism and the environment at the end of the previous stage. Following the transition to a new developmental stage, the intensity of functional metabolism decreases, while the value of plastic metabolism increases. In other words, during this period, a redistribution of total food energy occurs among various metabolic processes that hold the utmost importance for the development and growth of the organism at any given moment. In some cases, a simultaneous increase in the absolute indicators of functional and plastic metabolism occurs at the beginning of a new developmental stage, which may be associated with significant structural complications of the organism and quite substantial changes in their biology. For instance, at the developmental stage marking the onset of mixed feeding, the surge in functional metabolism may be linked to an increase in the organism's energy expenditure on food searching, capture, Digestion, and so forth.

Throughout each individual developmental stage, plastic metabolism typically predominates over functional metabolism, the magnitude of which declines toward the end of the stage. This pattern is reinforced by the potential exhaustion of both morphological and functional capacities specific to that stage of the organism's development. Consequently, a necessity arises for new morphophysiological and biochemical transformations to propel the organism into the next developmental stage.

Alongside the examination of the GENERAL PATTERNS OF early fish ontogeny under commercial aquaculture conditions, the embryonic and larval periods of development are of exceptional importance, justifying their separate consideration.

A fertilized egg is already a developing embryo that progresses through a series of specific developmental stages. At the fertilization stage, following the penetration of a spermatozoon into the egg, complex transformations and rearrangements of its contents are observed. Within 12–15 minutes post-insemination, the eggs of certain fish species (such as cyprinids and sturgeons) become adhesive, readily attaching to the substrate. Egg membranes swell in water, becoming transparent and tough. The eggs increase significantly in size. Specific gravity of the eggs decreases. The perivitelline space fills with liquid. The surface layer of Cytoplasm shifts toward the opposite side of the animal pole. This stage concludes with the fusion (combination) of the female and male pronuclei. The transition to the next developmental stage—Cleavage—begins with the appearance of the first cleavage furrow on the eggs, forming two blastomeres. At this stage, each egg sequentially divides into an increasing number of blastomeres, reaching the blastula stage by its end. Next, the embryo enters a new developmental stage—Gastrulation—during which the embryonic body and its yolk sac begin to form. At the subsequent stage of embryonic development, neurulation takes place. The embryo forms a neural tube, which serves as the primordium of The Nervous System.

Subsequently, the body shape of the embryo changes, the caudal region demarcates, the Brain divisions form, Sense Organs and hatching glands are laid down. The digestive and excretory systems begin to form in the embryo, The Heart and blood vessels develop, gill pouches are laid down, the Liver primordium appears, Muscle fibers and Blood Cells are formed, and vacuolization of chordal cells begins. The embryo's body segments into individual sections, and a fin fold appears on it.

Having passed through a series of developmental stages, the embryo already possesses a functioning heart and circulatory system, as well as a nervous system, and responds to external stimuli. At this point, the strength of the egg membranes is weakened by the action of hatching Enzymes, and the embryos break free from them.

The embryonic period concludes at the stage when the hatched free embryos (eleutheroembryos) complete their development and transform into larvae.

From the very First stage of its development, the embryo enters into close contact with the surrounding environment. Water temperature, gas regime, pH, salinity, osmotic pressure, and illumination exert a significant influence on embryonic development (Embryogenesis). For each fish species, embryogenesis occurs within specific temperature ranges, distinguishing the temperature optimum, temperature threshold, and temperature maximum. The temperature at which embryogenesis proceeds normally is termed the temperature optimum. An increase in temperature accelerates embryogenesis, whereas a decrease slows down this process.

The temperature below which embryogenesis ceases is known as the temperature threshold. However, brief exposure to threshold temperatures does not cause embryonic mortality. Once the optimal temperature is restored, embryogenesis resumes and proceeds normally. The temperature above which embryogenesis halts and the embryo dies is called the temperature maximum. Understanding the thermal limits for normal embryogenesis is crucial not only for more accurate forecasting of future fish productivity but also for managing artificial fish breeding and incubation processes effectively.

The gas regime plays an equally vital role in embryonic development. In the absence of dissolved oxygen, the embryo dies of asphyxiation. Reduced oxygen levels in the blood slow down embryonic growth. If oxygen deficiency coincides with elevated temperatures (above the optimum), embryonic development becomes abnormal, leading to deformities and subsequent mortality. A similar phenomenon is observed at high CO₂ concentrations, which can occur when incubating a large volume of eggs in a small amount of stagnant water. In this case, CO₂ shifts the active reaction of the medium toward the acidic side, disrupting gas exchange within the embryonic cells.

Embryos respond differently to temperature, gas conditions, salinity, light, and mechanical stimuli across various developmental stages. Some stages are highly sensitive to sudden fluctuations in abiotic environmental factors—leading to an increase in deformed embryos and mortality—while others are less vulnerable. Heightened sensitivity to abrupt changes in the aquatic environment typically manifests during the early stages (fertilization to gastrulation) and around the hatching phase.

Consequently, egg incubation must be conducted strictly under conditions where environmental factors support normal embryogenesis. Under such optimal conditions, the embryo grows and develops steadily by utilizing the nutrient reserves of the yolk.

The freshly hatched embryo, or pre-larva, leads a passive lifestyle initially. It is nourished by the nutrient reserves stored in the yolk sac, which serves as a temporary organ. The yolk sac also functions as a provisional respiratory organ for the pre-larva. As the pre-larva grows, the yolk sac gradually diminishes in size. Shortly before its complete resorption, the embryonic period ends and the larval period begins. The pre-larva transforms into a larva, shifting to a mixed feeding strategy. At this stage, the larva consumes both the remaining yolk reserves and exogenous food from the surrounding environment.

The yolk sac in the larva is soon fully resorbed, and the organism transitions entirely to external feeding. The larva grows and develops, eventually transforming into a fry that outwardly resembles an adult fish.

The duration of the larval development period, like the embryonic one, varies among fish species—ranging from a few days to a month. For each species, this duration increases or decreases depending on water temperature and other abiotic and biotic environmental factors.

Understanding the stages of fish development allows researchers to study the pathways and functional mechanisms by which anadromous, semi-anadromous, and freshwater fish populations adapt to modern conditions of integrated aquatic resource use, leveraging their maximum physiological plasticity in response to aquatic habitat changes.

Pre-larvae consume yolk sac reserves at varying rates, which largely depend on ambient temperature. Therefore, it is essential to understand how temperature influences growth and yolk utilization during the early ontogeny of fish.

Traditionally in ichthyology, embryonic growth and yolk utilization dynamics during the embryonic period are assessed using standard methods, such as measuring embryo length, body mass, and yolk mass. In some cases, researchers calculate the volume of the embryo and yolk or their projected surface areas.

Meanwhile, F.R. Hayes defined embryonic growth primarily as the synthesis and accumulation of protein within its Tissues, which serves as the foundation for increases in body mass and linear dimensions.

Studies by G.G. Novikov and N.D. Kuftina revealed that the protein mass growth of cod, lumpfish, and Atlantic salmon embryos during the Initial Stages of embryogenesis is minimal across various thermal conditions, which is attributed to a low rate of Protein Synthesis in the blastoderm. This view is supported by A.A. Neifakh and M.Ya. Timofeeva (1977) based on their respective research.

A noticeable increase in protein mass begins around the gastrulation stage (in cod and lumpfish) and at the Organogenesis stage in salmon. Subsequently, the Rate of protein growth accelerates, particularly with rising water water temperatures. Special studies have established that the higher the development temperature, the greater the daily rate of protein growth. However, by the time the embryo hatches from its membrane, its total protein mass decreases at higher temperatures.

Scientific research demonstrates that higher temperatures shorten the duration of embryonic development to a greater extent than they accelerate protein growth rates. This exact relationship is likely one of the primary reasons for the smaller final size of embryos upon hatching under elevated temperature conditions. This indicates that normal embryonic development occurs within an optimal thermal window, and any disruption in developmental dynamics can be interpreted as temperatures extending beyond optimal ranges.

Research has shown that embryonic development and vitality rely entirely on reserve yolk nutrients, with protein substances serving as the primary component (Neifakh A.A., Timofeeva M.Ya., 1977).

Unlike the curvilinear growth pattern of the embryonic body, whose rate fluctuates over time, the resorption of protein substances in the yolk occurs at a constant, linear rate. Given these regularities, the researchers concluded that The regulatory mechanisms governing embryonic body protein growth and the resorption of yolk protein reserves may be relatively independent of one another.

It is worth noting that the resorption rate of nutrient reserves, much like embryonic growth, increases with rising water temperature. Furthermore, the dynamics of yolk protein resorption align closely with The activity of Proteolytic Enzymes responsible for breaking down yolk protein molecules. Under normal conditions, the activity of acidic proteases remains stable throughout embryonic development; however, as water temperature rises, their activity increases correspondingly, as does the rate of yolk resorption.

Despite the fact that higher development temperatures accelerate yolk protein resorption, the total amount of protein resorbed during the period of encapsulated embryonic development actually decreases. In other words, The amount of residual protein remaining in the yolk at the hatching stage is directly dependent on temperature.

Consequently, the disproportionate changes between protein resorption rates and the duration of embryonic development at elevated temperatures result in hatchlings that are smaller in size and retain a larger protein reserve in the yolk. This indicates a disruption in normal embryonic development under conditions of thermal stress.

Of particular interest are studies investigating the dynamics of yolk protein content during embryonic development, which are assessed by analyzing protein changes in the whole egg. These dynamics are determined, on the one hand, by the rate of yolk protein resorption and, on the other, by the growth of the embryonic protein mass. Research by I.E. Moroz and V.P. Lukin (1973) demonstrated that at the onset of development, when embryonic growth is minimal, protein content is dictated solely by yolk resorption. As intensive protein growth begins, the overall protein content in the egg decreases, stabilizing toward the end of embryonic development.

Scientific findings by G.G. Novikov and N.D. Kuftina (1988) indicate that the overall utilization of yolk protein during embryonic development ranges between 10–40%, depending on the fish species and water temperature. During the encapsulated development of lumpfish, cod, and salmon, approximately 30–40% of the yolk protein is resorbed. However, direct embryonic growth utilizes 40–80% of this resorbed protein in cod and lumpfish, compared to only 20–30% in salmon. This demonstrates that the efficiency of protein growth per unit of initial egg protein mass during encapsulated development is 2 to 3 times higher in cod and lumpfish than in Atlantic salmon.

The residual protein is heavily utilized by the organism after hatching. Therefore, in fish with small eggs and minimal nutrient reserves, nearly all yolk sac reserves are depleted before exogenous feeding begins. Consequently, pre-larvae of these species cannot endure prolonged starvation, unlike salmonid pre-larvae, which possess much larger yolk reserves and expend only about 70–80% of their reserve protein after hatching.

Based on this, a high concentration of nutrient reserves in the egg is evolutionarily designed to sustain organismal development using endogenous resources after hatching until the transition to exogenous feeding.

Significant mortality occurs during the transition from the embryonic to the larval stage in the fish life cycle and aquaculture practices, a phenomenon backed by both scientific rationale and practical experience. This highlights the objective necessity of studying the regularities governing the transition from the embryonic to the larval state. Addressing this issue, it is useful to recall that fish ontogeny features two early periods—embryonic and larval—consisting of sequential, smoothly transitioning stages. Each stage is characterized by a complex of morphological, physiological, and biological traits that emerge through the Selection/27.html">Realization of Genetic heredity under specific ecological conditions. Every developmental stage possesses a specific set of organs and functions that vary depending on environmental factors. However, these changes have limits set by the organism's adaptive capacity, evolved over the species' history.

It has been proven that embryonic development in each specific case is governed by a multitude of factors that ultimately generate phenotypic diversity among offspring, broadening their adaptive range and ensuring optimal population size maintenance. Factors directly or indirectly influencing embryogenesis include: the age and physiological condition of broodstock; the Location of eggs within the Ovary and their degree of ripeness; the thermal regime during ovulation; sperm activity; the thermal regime during embryonic development, which affects embryonic differentiation and metabolism; as well as water mineralization, pH levels, and egg pigmentation.

The embryonic period ends with the embryo hatching from the egg, which marks the beginning of the larval period. Its first stage is endogenous feeding at the expense of reserve substances concentrated in the yolk sac.

At first glance, the moment of embryonic hatching appears to be a sudden, short-lived leap. In reality, however, hatching is a rather prolonged process accompanied by the accumulation of significant morphophysiological changes. This process consists of the embryo's preparation for hatching, the hatching event itself, and the subsequent adjustments that support the vital functions of the organism once it is free of the egg membranes. In each specific case, any of these phases can be shifted in either direction by The Influence of temperature, gas regime, light conditions, or mechanical factors.

One of the critical periods in embryonic development is the preparation for hatching, during which the embryo is most sensitive to external stimuli. This heightened sensitivity is attributed to several factors: the achievement of a species-specific level of organ system development, the intensification of embryonic motility, the increasing restriction the egg membrane imposes on the embryo's gas exchange, and the accumulation in the hatching Glands of the enzyme responsible for dissolving the egg envelope. Meanwhile, altering the temperature regime, the concentration of dissolved oxygen or carbon dioxide, and other environmental factors can disrupt the coordinated course of embryogenesis, causing a delay in hatching or, conversely, stimulating premature hatching, which is highly undesirable.

Among The most significant factors influencing hatching are water temperature, its active reaction (pH), dissolved gas content, and mechanical stimuli. A particularly important conclusion from several studies is that premature hatching in fish leads to the death of the embryos or larvae, as they leave the egg membrane too early.

In contrast, when hatching occurs after the complex of vital organs has already formed—namely, body segmentation is complete,

the HEAD and tail regions are separated from the yolk, a pulsating heart tube has appeared and pushes colorful (colorless) plasma, and neuromuscular motility has developed—the hatched embryos prove to be viable.

At the same time, under natural conditions, hatching may be delayed for various reasons, especially when temperatures drop; this indirectly alters the embryo's gas exchange, reduces the intensity of embryonic movements, and lowers the activity of the hatching enzyme.

It is important to note that the adaptations ensuring the survival of embryos outside the egg membrane must be broader in species that develop under conditions of sharp environmental fluctuations, particularly in shallow waters during early spring. Evidence suggests that late embryos and early larvae possess considerable phenotypic plasticity as an adaptive response. This plasticity is manifested in prelarvae and subsequently in prematurely hatched larvae through the accelerated development of vital organs—most notably the circulatory system, which facilitates the intensive metabolism required in their new environment. It has also been proven that the less developed the larva, the higher its developmental rate. This accelerated organogenesis continues until the complex of organs characteristic of normally hatched embryos is formed under optimal conditions.

The duration of the first developmental stage—the state of endogenous feeding—depends on the degree of embryonic maturity at the time of hatching. For some, the yolk-feeding period is prolonged; for others, it is short. Furthermore, because hatched larvae are more active, they consume the yolk reserves faster than sibling embryos still remaining within the egg membrane.

Thus, hatching can be viewed as a transitional leap, the duration of which depends both on species-specific adaptations and on the specific conditions of development.

The biological and ecological features of fish development at various stages constitute essential components of fisheries science, possessing both theoretical and practical significance. Today, an understanding of these developmental stages allows scientists to recommend that fish farms utilize anadromous fish juveniles prior to their downstream migration stage—a transition associated with complex physiological changes that prepare the organism for marine life, particularly in salmonids. To solve practical challenges in artificial fish breeding, studying the environmental factors that affect fish at different developmental stages is of paramount importance. All of this contributes to the Development of the most effective fish-rearing techniques.

The theory of developmental stages in fish has proven to be a vital theoretical foundation for designing biotechniques to rear the juveniles of commercially valuable species. It enhances the efficiency of artificial propagation by providing targeted information regarding the features and specificities of the spawning stage in fish development.

A thorough knowledge of fish reproductive biology, physiological traits, and ecological requirements during the spawning season is necessary to refine fisheries conservation measures, improve spawning ground amelioration works, develop eco-friendly methods for stimulating broodstock maturation, increase spawning efficiency in fish-rearing farms, and advance broodstock selection.

As noted above, the theory of developmental stages, formulated by V. V. Vasnetsov, is based on the concept of the inseparable, contradictory unity between the organism and its environment, viewing development as an adaptive process. According to this theory, development is simultaneously continuous and evolutionary, yet discontinuous and saltatory (proceeding in leaps), with the entire life cycle unfolding in stages. Over a certain period of time—termed a stage—the biology of the developing organism remains relatively unchanged. V. V. Vasnetsov wrote: "Each stage differs from the others by its specific structure, physiology, and biology, with both structure and physiology being adapted to that particular biology and to those specific conditions." In other words, each stage is characterized by a specific system of adaptations. Changes within a stage, whether quantitative or qualitative, do not cross a certain threshold or alter the adaptive system; rather, they prepare the developing organism for such a transition. Upon advancing to a new developmental stage—a transition that occurs abruptly—the entire biology of the organism changes significantly, while biologically similar stages are grouped into developmental periods.

Of considerable interest in this context is the timeframe during which fish spawning occurs. It is well known that many fish species select specific sites for spawning, undertaking Migrations to designated areas of water bodies. Lithophilic fish travel to spawn on submerged vegetation, even though their feeding grounds are located elsewhere. Fish that release pelagic eggs in fresh water move into currents to spawn. Some species feed in coastal marine zones but migrate to the pelagic zone for reproduction. The Pacific herring and several other pelagic species approach the shoreline to breed. The spawning environment of anadromous sturgeons, salmonids, and other species differs significantly. The migration of fish to an ecologically distinct spawning ground is preceded by functional and interrelated morphophysiological restructuring of the organism.

However, sharp differences between spawning and feeding grounds are not characteristic of all fish; consequently, pre-spawning changes may be weakly expressed or externally unnoticeable.

There are also fish species that do not change their habitat location during the reproductive period, yet they still undergo significant morphological changes driven by the specifics of their reproductive biology. In the process, various organs and tissues undergo morphological and functional transformations, and new organs and functions emerge. In ovoviviparous, viviparous, and brood-caring fish, the gut, reproductive, and other organs are modified to supply developing embryos not only with oxygen but also with nutrients. Through various novel structures developing in the pre-spawning period and functional shifts in various organs, the fish enters into qualitatively new relationships with the abiotic and biotic environment—particularly with members of the opposite sex and its offspring—leading to a profound transformation of its biology.

A fish's relationship to food has a substantial impact on its organism during the spawning period. Many fish species cease feeding during reproduction. These include fish that migrate to spawning grounds devoid of food sources for adults, classic examples being salmonids and sturgeons. Certain species that incubate eggs in their mouth or gut also fast during this period. In other fish species, the composition of food organisms changes due to the diverse fauna found in spawning versus feeding grounds. Feeding rhythms also shift in accordance with the broader biological transformations of spawning fish. Cessation of feeding, dietary shifts, and altered feeding rhythms lead to changes in metabolic patterns. It is well known that metabolism during reproduction is further altered by changes in the activity of Endocrine glands. Reserve substances accumulated during the feeding period are channeled into the formation of sex products—and in viviparous fish, into the Formation of the embryos themselves—exemplifying generative metabolism.

When delineating developmental stages, great importance is attached to shifts in foraging methods, feeding habits, and metabolism.

The pre-spawning and, particularly, the spawning period are characterized by changes in shoaling behavior, the formation of specialized spawning aggregations, and the development of complex behavioral reactions during the mating season. This indicates that spawning fish enter into specific relationships unique to this period of life.

Consequently, fish reproduction cannot take place within the framework of prior adaptations. One way to resolve this contradiction is through organismal restructuring. The changes occurring in a fish's body as it enters spawning condition represent a shift in the adaptations required to secure reproductive processes. This restructuring is prepared gradually and culminates abruptly right before spawning begins.

The saltatory (leap-like) nature of biological change is one of the key indicators that an organism has transitioned to a new stage of development.

Because spawning fish differ in physiological state, Morphology, and behavior, enter into specific relationships with the abiotic and biotic environment, and perform a specialized function, this segment of ontogeny is distinguished as a qualitatively new, spawning stage of development.

Along with these General Principles, a more detailed analysis of this stage in monocyclic and polycyclic fish is advisable, taking into account their specific reproductive traits.

Monocyclic fish—striking examples being Pacific salmon, eels, and certain gobies—die after spawning; in other words, reproduction occurs only once in their lifetime.

In polycyclic fish, which spawn multiple times throughout their life span, ontogeny involves several cycles of gonad development.

Scientists warn that repeated spawning should not be confused with seasonal cyclicity, which reflects the environmental characteristics of all developing organisms. It is well known that each population reproduces during a specific season of the year. However, not every individual reproduces upon the arrival of that season. Only physiologically prepared individuals initiate spawning, which indicates that the spawning stage is only relatively linked to seasonal cyclicity.

Meanwhile, it is important to note that spawning in both monocyclic and polycyclic fish concludes in fundamentally the same way: with the completion of the gonad development cycle and the fish's transition into a new biological state. For the former, this means death, whereas for the latter, it marks a transition to feeding and recovery. In other words, this and other facts indicate that there are no fundamental differences between monocyclic and polycyclic fish.

However, in some species or individual fish, the timing of sexual maturation can be shifted under the Influence of the environment. Determining the specific aquatic environmental factors that drive this timing is crucial, as it is closely linked to the development of methods

for accelerating or decelerating sexual maturation processes—in other words, for managing fish reproduction.

At the same time, some factors that promote fish maturation are already known. The completion of maturation and the transition of spawners into a running state normally occur under the influence of aquatic environmental factors that determine the ecological Specificity of the species' spawning grounds. Thus, the preparation and transition of fish into the spawning state, like the entire developmental process, is adaptive in nature.

It is known that under artificial propagation conditions, physiologically active substances are used to transition certain fish species into a running state. However, this method of physiological or hormonal stimulation of fish maturation has limitations, as injections yield positive results only when the fish have reached a sufficiently high degree of ripeness. To achieve this ripeness, specific ecological conditions must be provided for the fish. These two approaches are considered complementary and today are viewed as an ecophysiological method for gamete maturation.

Consequently, the spawning stage of development is of particular importance in completing the ontogenetic cycle and ensuring the unity of its various stages.

Clearly, the Theoretical Aspects of modern fish culture are closely intertwined with the practical operations of the corresponding fisheries sectors. Based on this, it is advisable to identify and articulate the specific components that define the Practical significance of the theory of developmental stages in fish farming.

- Understanding the stages of fish development and the specific features of their biology under both natural and artificial conditions makes it possible to determine the needs of fish at each developmental stage. This, in turn, helps identify shortcomings in artificial propagation biotechnology, which is essential for rationalizing fish breeding. The theory of developmental stages provides a biological rationale for various links in fish-breeding biotechnology and enables biological control over fish development in both natural and artificial environments.

- Identifying the developmental stages within populations inhabiting a body of water helps us understand the heterogeneous structure of that population and The Diversity of its needs.

- Determining the developmental stages of commercial and low-value fish species inhabiting the same water bodies clarifies to what extent and at which developmental stages low-value fish exert an additional burden on the food supply of commercial fish, thereby refining our understanding of interspecific relationships among fish.

- The theory of developmental stages requires studying the food supply with consideration for different consumers at each stage of development, thereby making research into fish feeding resources more specific. Knowing the characteristics of each developmental stage allows for more reliable forecasting of commercial fish returns. A deficit of specific food at any developmental stage can significantly reduce the size of the commercial stock.

- Shifts in the timing of sexual maturation depending on the conditions under which the fish develop can only be detected with a clear understanding of the fish's developmental stages.

- Knowledge of developmental stages is essential for uncovering the patterns underlying population dynamics.

It should be noted that to date, not all questions related to stage-by-stage fish development have been sufficiently studied. In particular, the comparative patterns of developmental stages across different ecological groups of fish have not yet been established. The specific features of transitions from one stage of sexual maturation to the next, starting from the moment embryos hatch from their membranes, remain unclear. Comprehensive information regarding The impact of various environmental conditions on developmental stages and the transition from one stage to another is lacking. It is also important to determine the eco-Morphological Characteristics of various organ systems at different developmental stages. In other words, studying the functions of all organs and their changes throughout the organism's life cycle from conception to death is highly relevant. One of the most critical issues in advancing the theory of developmental stages is the poorly understood problem of the transition into the stage of sexual maturation in fish. Of particular note are questions concerning The Effect of environmental factors on the rate of fish sexual maturation.

Meanwhile, there is still no clear consensus on what point in a fish's development should be considered the beginning of the sexual maturity period. Some researchers believe this boundary should be the onset of the first spawning, and the period itself should be called the reproductive period. Another view is that this period comprises one or more spawning stages along with several other stages, depending on the ecological characteristics of the fish: the spawning migration stage, the post-spawning downstream migration stage, the embryo-brooding stage, parental care, and so on.

The ecological traits of fish are so diverse that one can assume the existence of multiple developmental stages in the interval between successive spawnings in some species. In sturgeons, such an interval lasts for several years, during which the fish likely passes through multiple developmental stages, undergoing substantial changes as it prepares for new reproduction. In single-spawning fish, the reproductive period marks the end of the fish's life. However, the onset of this final life period is associated with quite significant morphofunctional and ecological changes in the fish. It is known that the beginning of each developmental period coincides with the onset of a developmental stage. In Pacific salmon, this makes the reproductive period The final stage of their existence.

In many fish, spawning and feeding stages alternate throughout the reproductive period. Each spawning stage is characterized by Structural and functional features specific to the ecology of that stage. If spawning is extended over time and the fish's biology does not change significantly in the intervals between releasing individual portions of eggs, such spawning is grouped into a single stage.

Obviously, under strictly defined conditions, multiple-spawning fish go through an equal number of spawning and feeding stages throughout their lifetime.

Delineating the spawning stage in fish development accurately reflects the biological uniqueness of reproductive processes, enabling a deeper and more detailed understanding of adult fish. Knowledge of the developmental stages within a period—and particularly the reproductive period—allows us to address important issues in fish culture, such as the causes of spawning delays, optimal spawning frequencies for each species under specific conditions, ideal environmental factors promoting successful spawning, and methods for accelerating this process.

The theory of developmental stages helps us understand the adaptive nature of organisms at every moment of their individual lives and species history, identifies the organism's needs at each developmental stage, and thereby provides the key to managing this development.

The Life Cycle of fish includes periods characterized by heightened sensitivity to external factors. These states are regarded as Critical Periods of development in fish, which are associated with the impact of negative factors on specific Components of the life cycle.

Critical periods of development correspond to specific moments in the organism's differentiation. Upon reaching these moments, embryos and larvae carrying morphophysiological defects—either inherited from parents or caused by adverse conditions during embryonic and postembryonic stages—are no longer capable of further development and perish. This typically occurs during the most crucial stages of early ontogeny, especially at the onset of organ and system functions. Because defective individuals may exhibit varying types of defects and degrees of organ impairment, those with the most severe defects cease development and die earlier, during the early critical periods. Others with lesser impairments perish later, while those with minor defects may bypass the critical developmental stages altogether. However, subsequently under adverse biotic and abiotic environmental conditions, elimination occurs primarily among these latter individuals. Thus, critical periods of development can be briefly defined as periods when morphophysiological defects are realized, and death is the external manifestation of these critical periods.

During critical developmental periods, the organism maintains specific relationships with its environment, but these relationships only partially, rather than entirely, determine these periods.

There are several such critical periods during the embryonic and post-embryonic development. For many fish species, critical periods include the stages of late gastrulation and early organogenesis, while for some species, hatching is also a critical stage.

In some marine pelagophilic fish, the late cleavage and blastula stages can apparently also be considered such a period.

The larval stage of development also reveals several critical periods, with different groups of fish species experiencing them at various developmental stages. During the larval stage, critical periods do not necessarily coincide solely with the end of yolk resorption and the transition to exogenous feeding.

Given its theoretical and practical relevance, it is advisable to examine the critical periods of the larval stage in individual fish species.

Some researchers believe that the critical period for sturgeon is the transition of larvae to active feeding. It is during this period that mass mortality of larvae is recorded in artificial sturgeon breeding. This phenomenon is attributed to untimely feeding and a mismatch between food quality and the larvae's requirements.

However, another perspective exists, based on the fact that larval mortality is quite frequently observed despite favorable feeding conditions, and furthermore, it begins even before the larvae transition to active feeding. Therefore, it is believed that larval mortality is caused by the poor quality of Gametes used in artificial reproduction and unsatisfactory incubation conditions. This conclusion was confirmed by special studies establishing that sturgeon larvae dying during this period were characterized by delayed growth in length and weight, as well as various structural defects: hunched backs, snub-nosed rostrums, reduced liver size, various gut abnormalities, often combined with ascites of the Abdominal cavity AND Stomach. All this indicates that the developmental period of sturgeon larvae just before and at the beginning of active feeding can be considered a critical period of development. In herrings, the critical period of development is the end of yolk sac Nutrition and the beginning of active feeding. Salmonids also exhibit a distinct critical period of development at the time of final yolk resorption and the onset of active feeding.

As for cyprinids, they experience two periods of high larval mortality. The first period of carp larval mortality is observed 5–7 days after hatching, after they have completely switched to exogenous feeding. The overall timeframe for this mortality extends up to the 10th–12th day after hatching. The dying larvae lagged significantly in growth and development behind their peers; they either did not feed at all or fed very little. They also showed an absence or poor development of intestinal mucosal folds. It is assumed that during this period, developmental defects in the digestive organs become manifested.

The second period of high mortality in carp larvae is usually recorded from the 19th to the 27th days after hatching. Prior to death, these larvae also significantly lagged in development and growth. Most of them failed to develop a dorsal fin, showing only an accumulation of mesenchymal cells in its place, while some larvae showed only the emergence of the first rays. In contrast, normal larvae exhibited fully formed fins, except for the pelvic ones. The growth difference is sometimes so pronounced that larger larvae often swallow smaller ones or nibble on their tails.

At the same time, larval mortality during these peaks could not be attributed to a shortage of suitable food or other external causes.

It is believed that the direct cause of larval mortality during these periods of high mortality is internal rather than external. This cause is considered to be the poor quality of the larvae themselves, which in turn depends on the poor quality of the gametes produced by the broodstock. To some extent, larval survival could be affected by sharp temperature drops or increases, which are quite frequently observed during larval rearing. Therefore, the two considered periods of high mortality in carp larvae can be considered critical periods of development.

Two periods of high mortality have also been identified in bream larvae. The first occurs on days 12–17, and the second on days 20–22, which are critical for bream larvae.

During the larval stage, roach (taran) also exhibits two main critical periods of development, similar to bream, with the only difference being that the second period is most pronounced in bream, whereas the first is most pronounced in roach; moreover, the duration of these periods in roach is, on average, slightly longer.

Completely different timeframes for high mortality periods are observed in grass carp larvae. The mass mortality period for this fish species lasts generally from the 1st to the 6th day after hatching, with peaks occurring on days 1–2 and days 4–5. It has been experimentally established that the elimination of grass carp larvae due to non-viable individuals occurs mainly during the transition to active feeding—yolk resorption in them ends on the 5th–6th day after hatching.

Consequently, it is believed that grass carp larvae have two critical periods of development. One covers mainly the first 2 days after hatching, and the second encompasses the end of yolk resorption and the onset of active feeding, i.e., days 4–6.

It is believed that the development of both the embryo as a whole and its constituent parts—meaning organs and tissues—consists of a small number of stages characterized by specific morphophysiological features. Each of these stages begins with a relatively short critical period, followed by visible developmental processes, namely growth and differentiation. The main feature characterizing critical periods is the high sensitivity of embryonic cells to external factors, caused by significant regulatory activity during these periods.

According to current concepts, periods of high developmental sensitivity in fish appear at turning points of ontogenesis, prior to the most important morphological differentiations. They are characterized by a high sensitivity of the organism to the NEGATIVE IMPACT OF external factors, a decrease in the organism's viability, its overall metabolism, and other physiological properties, which show that these changes are not only the cause of high sensitivity during these developmental periods of the organism, but are themselves a consequence of Cell damage. These periods are also characterized by enhanced respiration and a reduced growth rate.

Various researchers identify a varying number of periods of high sensitivity in fish embryonic development—from 3 to 8, which is due to species-specific features and environmental conditions.

Significant morphological and physiological defects inherited by larvae from their parents and acquired during embryonic development under adverse conditions are usually manifested during critical periods of development when certain organ systems begin to function. Critical periods are a species-specific or population-level trait rather than an individual one, as they apply to The population as a whole. In progeny originating from high-quality eggs and under normal developmental conditions, critical periods do not manifest. However, the elimination of low-quality individuals occurs not only during critical periods; under the deterioration of any biotic or environmental conditions, it is precisely these individuals that perish first, and such mortality apparently plays a major role in the dynamics of generation survival.

The degree to which critical periods manifest depends, on the one hand, on the qualitative state of the organism and, on the other, on environmental conditions; under unfavorable conditions, they may appear more clearly. Since it has been established that the rate of development depends on environmental conditions, the timing of critical periods also depends to some extent on them. Critical periods do not manifest in progeny that developed from high-quality gametes and under favorable environmental conditions, and the survival rate of such progeny is sufficiently high; therefore, they should be considered a population trait rather than an individual one.

There is not just one, but several critical periods of development in early ontogenesis, and during the larval stage, critical periods do not necessarily coincide exclusively with the end of yolk resorption and the transition to active feeding.